This invention relates to spray coating a moving substrate. More particularly, it relates to providing a uniform thickness or quantity of atomized particles on a substrate that may be moving at varying speeds.
Applying coatings such as oils and waxes by directly spraying or by electrostatic deposition on a continuously unwinding strip from a coil is well known. Electrostatic deposition of such sprayed materials has become increasingly popular because it provides efficient use in applying uniform thicknesses of coating materials. Whether a material is applied by direct spraying or electrostatic deposition, however, it has been a problem to apply a uniform thickness of coating to a substrate moving at varying speeds. Thus, during start-up or shutdown of a coating line, for example, the quantity of material delivered to a unit area of sheet would vary unless changes were also made in control of atomizing and/or spraying the material.
A number of proposals have been made to synchronize changes in control of the rate of deposition of material with changes in the speed of the substrate. For example, Landgraf U.S. Pat. No. 2,994,618 proposes that the velocity of a secondary air supply which carries atomized material into the deposition chamber be varied as the speed of the moving substrate varies. Nishikawa et al U.S. Pat. No. 3,726,701 controls both the air flow carrying the atomized particles and the potential in the electrostatic charging zone as the speed of movement of the substrate varies to control the uniformity of deposition of material.
An additional problem has been to apply very thin coatings of materials uniformly to a moving substrate. It is evident that the thinner the desired coating, the greater the percentage of difference in coating thickness with any variation at all in delivery of material to the substrate. The severity of the problem and the importance of finding a solution can be illustrated with reference to just one example. Aluminum sheet manufacturers supply millions of pounds of coiled sheet each year to the can industry for making can bodies and can ends. The sheet for making can ends is coated with a thin layer of Dioctyl Sebecate (DOS), a synthetic lubricant, which is applied to prevent the generation of surface defects during shipping and handling. A substantial portion of the can end material is coated on both sides or one side by the can maker with an organic coating before being formed into can ends. The organic coating is applied to serve as a barrier between the metal and a food or beverage product in the can. The organic coating is applied over the DOS layer, but if the DOS layer is too thick, the organic coating will not adhere properly to the sheet. For this reason, can manufacturers specify that there be no more DOS material on the sheet than that which is absolutely necessary to protect the surface finish. Typically, it is applied at a nominal weight of 1.0 mg/ft.sup.2. As an indication of how thin a coating of DOS of that weight is, it would take only 55.4 ml to cover an area the size of a football field with such a coating weight. It is apparent that the difficulty of uniformly applying such a thin coating is compounded when the speed of the substrate increases in starting up a coil or slows down near the end of the coil. Consistent and uniform application of thin coatings of DOS lubricant to can end stock has been a major problem for metal manufacturers because of the large quantities of metal used in this application.
Thus, it may be seen that it is desirable to compensate for changes in speed of a moving substrate in the application of an atomized sprayed coating of materials thereon.